Current methods of identifying agents useful as antiparasitic drugs are inadequate and the use of molecular biology to this end has been limited. Nearly all molecular biology research done globally in parasites is devoted to antigen characterization and production with a goal of vaccine development. Discovery of drugs effective against many parasites is hampered by the inability to recover them in sufficient quantities to isolate proteins for study and screening. Many helminthic parasites grow poorly in convenient laboratory animals and are uniformly intractable to culture. Obtaining them from natural hosts, even in small quantities, is tedious and expensive.
Accordingly, using present methods, it is difficult to study parasites and screen compounds which may be useful as drugs against them. The absence of an effective method of finding antiparasitic drugs hinders efforts to successfully control or eradicate many diseases. The diseases or groups of diseases described generally as helminthiasis are due to infection of the animal with parasitic worms known as helminths. Helminthiasis and helminthosis are prevalent and may lead to serious economic problems in sheep, swine, cattle, goats, dogs, cats, horses, poultry and man. Among the helminths, the groups of worms known as nematodes, trematodes and cestodes cause widespread and often times serious infections in various species of animals including man. The most common genera of nematodes, trematodes, and cestodes infecting the animals referred to above are Dictyocaulus, Haemonchus, Trichostrongylus, Ostertagia, Nematodirus, Cooperia. Bunostomum, Oesophagostomum, Chabertia, Strongyloides, Trichuris, Fasciola, Diorocoelium, Enterobius, Ascaris, Toxascaris, Toxocara, Ascaridia, Capillaria, Hererakis, Ancylostoma, Uncinaria, Onchocerca, Taenia, Moniezia, Dipylidium, Mecastrongylus, Macracanthorhynchus, Hyostrongylus, and Strongylus. Some of these genera attack primarily the intestinal tract while others inhabit the stomach, lungs, liver and subcutaneous tissues. The parasitic infections causing helminthiasis and helminthosis lead to anemia, malnutrition, weakness, weight loss, unthriftiness, severe damage to the gastro-intestinal tract wall and, if left to run their course, may result in death of the infected animals Gastrointestinal nematodes, such as Haemonchus contortus, are important pathogens of sheep and cattle. These species are representative of a large number of economically significant ruminant parasites. These parasites are examples of those which present the above described problems in drug discovery.
One characteristic shared by many helminthic parasites is their dependence on a single pathway for energy generation. For intestinal parasites, glucose is an essential fuel. Pathways for energy generation in parasitic helminths differ from those in mammals and have long been proposed as targets for anthelmintic discovery. Nematodes which parasitize the gastro-intestinal tract are thought to be completely dependent upon the oxidation of glucose for the generation of energy. Gastrointestinal parasites are incapable of using amino acids or fatty acids as energy sources.
The present invention provides a new method of screening compounds in order to identify agents useful as antiparasitic drugs. The present invention provides an alternative to use of parasites as subjects to study and as sources of proteins. Thus, it is not necessary to collect large quantities of parasites, to grow them in vitro and to use them with convenient laboratory animals to study the parasites and screen for antiparasitic compounds.
The present invention provides a method of screening compounds by maintaining a target protein in a controlled biological system such that the effects on the target protein by the compound being screened can be easily observed. According to the present invention, it is possible to express specific parasite proteins in biological systems which will allow high volume screening for novel compounds with specific mechanisms of action. Furthermore, according to the present invention, the action of a compound on a target protein can be observed independent of other factors since the target protein is a metabolic pathway component which can be circumvented in control tests by addition of a substance capable of being metabolized by subsequent pathway components.
Potential drug targets include metabolic enzymes known to be the site of action of available antiparasitic drugs as well as those which, based on an understanding of parasite physiology and biochemistry, represent novel mechanisms of actions for selective chemotherapeutic intervention. Leads that differentiate between host and parasite on a mechanistic basis provide the most likely candidates for successful development. Through the identification characterization and cloning of potential drug targets and their expression in an appropriate system for the production of material for biochemical study and for the development of high volume mechanistic screens the difficulties associated with screening agents in parasite organism systems are avoided. Cloning important enzymes and structural proteins into systems permits high volume screening of target parasite species which are highly intractable to culture in vitro, and whose acquisition on a regular basis is expensive and difficult. Furthermore, in contrast to other chemotherapeutic targets, very little is known about host-pathogen differences, other than pharmacological, for parasitic organisms. A critical axiom of experimental chemotherapy is that exploitation of such differences is the key to successful treatment.